The present invention relates generally to screen printing and, more particularly, to a method and apparatus for pre-registration of printing screens in a multi-color screen printing operation.
Recently, the screen printing industry has undergone enormous growth, due primarily to the popular appeal of shirts, caps, jackets, and the like bearing printed designs and logos. Screen printing of designs and logos onto articles of clothing is a common practice. Most simply, an article of clothing, such as a T-shirt, is placed upon a flat surface, and a print screen is positioned on the T-shirt. Thereafter, the printing dye or ink is transferred through the screen and onto the T-shirt by means of a roller or squeegee. When a multi-color design is to be printed onto the T-shirt, a set of individual print screens are used, with each print screen being used for application of a different color.
A common problem associated with multi-color printing is the alignment or "registration" of consecutive print screens. Each subsequent print screen must be aligned with the previously different colors are printed sequentially over one another, thereby requiring proper registry between the subsequent screens to achieve a neat, clear, multi-color pattern which can be rapidly and repetitively reproduced in a cost-effective manner.
In custom printing shops, such as those specializing in printing indicia onto T-shirts, as well as other relatively low volume screen printing enterprises, the number of articles to be printed with a particular design at one time is typically very small. Hence, it is preferably that the screen printing machines used on this specialized industry be highly adaptable, and capable of quick change-overs for printing different styles and shapes of articles in a wide variety of multi-colored patterns. Heretobefore, conventional screen printing machines or "screen printers" have not been adapted for pre-registration of print screens. Conventional print screen registration techniques, associated with even the most expensive printing machinery, are complicated, time consuming and manually labor intensive.
The related art to which the present invention is directed teaches many different types of rotary screen printers. As used herein, the term rotary refers to angular displacement of the print screens about a vertical axis when multi-color designs are to be printed. The aforementioned related art generally includes a four-color, one-station screen printer; a four-color, four-station screen printer, a six-color, one-station screen printer; and a six-color, six-station screen printer. All of the foregoing conventional rotary screen printers have the common problem associated with the positioning (i.e. registration of the multi-printing screens. Such multi-color rotary screen printers include multiple screen frame holding assemblies for supporting the print screens bearing different color images. The screen frame holding assemblies extend from a rotary table, and are generally spaced in equal angular fashion about the vertical axis of rotation. One or more platens are disposed below the screen frame holding assemblies. The platens can be fixedly or rotatably supported relative to the rotary table. Generally, the platens are also spaced in equal angular increments about the rotational axis.
Conventional screen frame holding assemblies are equipped with one or more clamping assemblies for securing the print screens thereto. In particular, the print screens are inserted in the clamp assemblies and are thereafter bolted and/or clamped in a desired alignment. However, it is not uncommon for the screen to shift during the printing operation so as to fall out of the desired registration. Printed images in which the color separations are not in accurate alignment results in articles which are not suitable for sale. Furthermore, the initial manual registration procedure is tedious, time-consuming and inefficient, especially in light of the slow output rate and high labor factor associated with small volume runs. Once the desired registration is lost, it is difficult to re-align or re-register the first color pattern on the article subsequent colored patterns.
FIG. 1 is a perspective view of a print screen 10 installed in rear clamp assembly 12 of a pivotable print screen frame holding assembly 14 associated with most conventional screen printers. While a single print screen 10 is illustrated, it is to be understood that the alignment problems to be hereinafter described in greater detail are inherent to all conventional screen printers having any number of pivotable print screen frame holding assemblies 14 and/or any type of clamping assemblies. Print screen 10, the fabrication of which is well known, typically includes a wooden border or frame 16 with a partially imperforate screen 18 mounted tautly to a lower portion thereof. As will be detailed hereinafter in greater detail, and is well known in the screen printing art, print screen frame holding assembly 14 is pivotally movable in a generally vertical plane between an upward "storage" position and a downward "printing" position. In the printing position, screen 18 engages an article which is located on a platen (not shown). The article on which the design or image is printed can include any garment-type products such as shirts, jackets, pants, caps and the like. Furthermore, any relatively flat stock (e.g. window stickers, bumper stickers, signs, decals, etc.) can also be similarly screen printed. Each print screen 10 includes a perforate portion in the shape of a desired image or design 20 through which the printing dye or ink is rolled or squeezed in accordance with application methods known in the screen printing industry.
Print screen 10 is installed in conventional clamp assembly 12 so as to be slid inwardly such that an outer edge surface 22 of frame 16 abuts a clamping surface 24 of rear clamp device 26. Fasteners 28 are adapted to be rotated to tighten a locking member (not shown) into engagement with an upper surface 30 of print screen 10. Typically, clamp assembly 12 further includes side clamps 32 consisting of a C-shaped clamp arm 34, pivotable link 36 and pivot joint 38. Side clamps 32 are provided on opposite ends of rear clamp device 26 and are adapted to pivot inwardly to generally surroundingly capture frame 16. Generally, side clamps 32 are not pivoted into position until print screen 10 has been at least partially aligned relative to the platen such as by fasteners 28 being tightened to secure print screen 54 in rear clamp 26. Link 36 is provided with a slot 40 to accept conventional screens of virtually any width dimension commonly used in screen printing applications. Fasteners 42 extend through slot 40 and an upper portion of C-shaped clamp arm 34 to engage upper surface 30 of frame 16 when tightened. The structure illustrated in association with clamp assembly 12 is exemplary in nature to show the complicated manual alignment technique associated with a registration of a print screen in a conventional print head.
In general terms, the screen printing process, heretobefore known in the industry, is accomplished by bringing (i.e., rotating) screen frame holding assembly 14, having screen frame print screen 10 secured thereto, to a position in alignment with the underlying platen. Print screen 10 is thereafter manually pivoted downwardly to the "printing" position. Screen 18 is urged firmly and abuttingly against the article and a roller or squeegee is used to transfer the dye or ink through screen 18 to print image 20 onto the article. Thereafter, print screen 10 is pivoted upwardly away from the platen to the "storage" position and, in multiple print screen frame holding devices, may be rotated to bring the next print screen over the platen. Thereafter, the above described sequence of steps is repeated for each successive print screen of a multi-color set.
In use, a set of printing screens is selected in accordance with the specific multi-color pattern to be printed onto the article. The print screens are typically designed in sets, with each set including that number of print screens corresponding to the number of different colors which the complete pattern requires. In particular, when several colors are to be printed on the same article, a separate print screen is used for each color, and the colors are printed sequentially over one another, thereby requiring proper registry between the various screens to achieve a neat, multi-colored pattern.
As is readily apparent from reference to FIG. 1, registry of color image 20 with the underlying article supported on the platen requires manual manipulation of print screen 10 relative to rear clamp 26 and side clamps 32 to produce proper alignment. Specifically, prior to tightening fasteners 28 and 42 against upper surface 30 of print screen 10, screen 18 must be properly aligned. The operator is required to manually align print screen 10 relative to clamp assembly 12 to align color image 20 into proper registration with the article. Typically, the operator is required to visually align image 20 which entails moving print screen 10 for and aft, sideways and/or angularly, relative to the rear clamp device 26. Side clamps 32 are thereafter pivoted into engagement with frame 16 of print screen 10 to provide additional rigidity and support. However, the original alignment of print screen 10 in clamp assembly 12 may be disturbed during normal printing operation thereby destroying the proper registration. Once this registration is lost, it is difficult to accurately re-align or "re-register" a particular color image on the article with subsequent color images. Although it is possible to re-register the subsequent multi-color printing screens, such a process is extremely inefficient and time consuming, thereby greatly increasing production costs. In particular, custom printing shops, specializing in relatively low volume runs, have herebefore been hampered by excessive set-up times for minimal order requirements.